When a wiper arm is hindered from operating in a wiper apparatus that swings on and wipes a surface of a windshield glass of a vehicle, an excessively large load acts on an output shaft of a wiper motor. This excessively large load can break a swing mechanism, a speed-reducing mechanism and other components in the wiper motor. In order to prevent the breakage of the mechanisms and parts, every part that constitutes the swing mechanism, the speed-reducing mechanism and the other components in the wiper motor is specified so as to have a strength that endures the excessively large load acting on the output shaft of the wiper motor. Thus, the above-mentioned factor in designing the wiper motor can increase a size and a weight of conventional wiper motor.
In view of the above-mentioned designing factor of the wiper motor which increases the size and the weight, the applicant of the present invention has put forth a wiper motor with a clutch mechanism integrated therein, to cut a load transmission between the output shaft and the swing mechanism or the speed-reducing mechanism, as disclosed in JP-2006-51835-A. In the wiper motor outfitted with this clutch mechanism, the excessively large load applied on the output shaft does not transfer to the swing mechanism and to a main body of the wiper motor. Consequently, it is possible to decrease the strength of the parts constituting the above-mentioned swing mechanism, the speed-reducing mechanism and the other components in designing the wiper motor, so as to decrease its size, weight and manufacturing cost.
However, even in a case that a wiping load acting on a wiper blade increases during an operation of the wiper apparatus and that a variation of the wiping load acting about an axis of the output shaft is within an assumed value, it is concerned that the wiping load can work the clutch mechanism to frequently cut the load transmission between the output shaft and the swing mechanism, etc. When the wiping load variation works the clutch mechanism as mentioned above, the wiper blade can stop wiping a wiping surface undesirably.
Further, when the wiper apparatus is turned on while accumulated snow and/or frozen ice is locking the wiper blade at a certain wiping angle on the windshield glass, the clutch mechanism cuts the load transmission between the output shaft and the motor body-side mechanism, and the motor body keeps operating the swing mechanism and the other components in the wiper motor. Thus, an operating noise that is continuously generated during the lock of the wiper blade can be an objectionable sound to a passenger in a vehicle.
Contrarily, in a case that an operating load and an engagement releasing torque of the clutch mechanism is too large, external force acting on the output shaft side can break the parts of the wiper motor with decreased strengths, prior to the operation of the clutch mechanism.